Resin-impregnated flexible graphite articles

ABSTRACT

Composites are prepared from resin-impregnated flexible graphite materials. Impregnated materials are compressed and cured at elevated temperature and pressure to form structures suitable for uses such as electronic thermal management (ETM) devices, supercapacitors and secondary batteries.

RELATED APPLICATION

This application is a continuation-in-part of co-pending applicationhaving Ser. No. 09/943,131 filed Aug. 31, 2001, entitled “LaminatesPrepared From Impregnated Flexible Graphite Sheets”, the disclosure ofwhich is incorporated herein by reference.

TECHNICAL FIELD

This invention relates to articles formed from resin-impregnated,compressed particles of exfoliated graphite (commonly referred to asflexible graphite), which are cured under heat and pressure and areuseful in applications such as heat transporters used in electronicthermal management (ETM), or current collectors for supercapacitors andsecondary batteries.

BACKGROUND ART

Compressed exfoliated graphite articles are known in the art, as arecomposite materials comprising resin-impregnated graphite sheets. Thesestructures find utility, for example, in gasket manufacture.

In addition to their utility in gasket materials, graphite compositesalso find utility as heat transfer or cooling apparatus. The use ofvarious solid structures as heat transporters is known in the art. Forexample, Banks, U.S. Pat. Nos. 5,316,080 and 5,224,030 discloses theutility of diamonds and gas-derived graphite fibers, joined with asuitable binder, as heat transfer devices. Such devices are employed topassively conduct heat from a source, such as a semiconductor, to a heatsink.

Graphite-based thermal management components offer several advantages inelectronic applications and can help eliminate the potential negativeimpacts of heat generating components in computers, communicationsequipment, and other electronic devices. Graphite-based thermalmanagement components include heat sinks, heat pipes and heat spreaders.All offer thermal conductivity comparable with or better than copper oraluminum, but are a fraction of the weight of those materials, andprovide significantly greater design flexibility. Graphite-based thermalmanagement products take advantage of the highly directional propertiesof graphite to move heat away from sensitive components. Compared totypical aluminum alloys used for heat management, the inventive graphitecomponents can exhibit up to 300% higher thermal conductivity, withvalues comparable to copper (˜400 watts per meter degree Kelvin, i.e.,W/mK) or greater. Further, aluminum and copper are isotropic, making itdifficult to channel the heat in a preferred direction.

The graphite material for use in this invention is graphite materialformed from compressed particles of exfoliated graphite.

The following is a brief description of graphite and the manner in whichit is typically processed to form flexible materials. Graphite, on amicroscopic scale, is made up of layer planes of hexagonal arrays ornetworks of carbon atoms. These layer planes of hexagonally arrangedcarbon atoms are substantially flat and are oriented or ordered so as tobe substantially parallel and equidistant to one another. Thesubstantially-flat, parallel, equidistant sheets or layers of carbonatoms, usually referred to as graphene layers or basal planes, arelinked or bonded together and groups thereof are arranged incrystallites. Highly-ordered graphite materials consist of crystallitesof considerable size, the crystallites being highly aligned or orientedwith respect to each other and having well ordered carbon layers. Inother words, highly ordered graphites have a high degree of preferredcrystallite orientation. It should be noted that graphites, bydefinition, possess anisotropic structures and thus exhibit or possessmany characteristics that are highly directional, e.g., thermal andelectrical conductivity and fluid diffusion.

Briefly, graphites may be characterized as laminated structures ofcarbon, that is, structures consisting of superposed layers or laminaeof carbon atoms joined together by weak van der Waals forces. Inconsidering the graphite structure, two axes or directions are usuallynoted, to wit, the “c” axis or direction and the “a” axes or directions.For simplicity, the “c” axis or direction may be considered as thedirection perpendicular to the carbon layers. The “a” axes or directionsmay be considered as the directions parallel to the carbon layers or thedirections perpendicular to the “c” direction. The graphites suitablefor manufacturing flexible graphite articles possess a very high degreeof orientation.

As noted above, the bonding forces holding the parallel layers of carbonatoms together are only weak van der Waals forces. Natural graphites canbe chemically treated so that the spacing between the superposed carbonlayers or laminae can be appreciably opened up so as to provide a markedexpansion in the direction perpendicular to the layers, that is, in the“c” direction, and thus form an expanded or intumesced graphitestructure in which the laminar character of the carbon layers issubstantially retained.

Graphite flake which has been chemically or thermally expanded and moreparticularly expanded so as to have a final thickness or “c” directiondimension which is as much as about 80 or more times the original “c”direction dimension, can be formed without the use of a binder intocohesive or integrated sheets of expanded graphite, e.g. webs, papers,strips, tapes, or the like (typically referred to as “flexiblegraphite”). The formation of graphite particles which have been expandedto have a final thickness or “c” dimension which is as much as about 80times or more the original “c” direction dimension into integratedflexible articles by compression, without the use of any bindingmaterial, is believed to be possible due to the mechanical interlocking,or cohesion, which is achieved between the voluminously expandedgraphite particles.

In addition to flexibility, the graphite material, as noted above, hasalso been found to possess a high degree of anisotropy to thermal andelectrical conductivity and fluid diffusion, somewhat less, butcomparable to the natural graphite starting material due to orientationof the expanded graphite particles substantially parallel to the opposedfaces of the material resulting from very high compression, e.g. rollprocessing. Material thus produced has excellent flexibility, goodstrength and a very high degree or orientation. There is a need forprocessing that more fully takes advantage of these properties.

Briefly, the process of producing binderless anisotropic graphitematerial, e.g. sheets, articles, web, paper, strip, tape, foil, mat, orthe like, comprises compressing or compacting under a predetermined loadand in the absence of a binder, expanded graphite particles which have a“c” direction dimension which is as much as about 80 or more times thatof the original particles so as to form a substantially flat, integratedgraphite article. Typically, the article formed is a flexible,relatively thin (i.e., 5 mm or less) sheet, although thicker articlesare also capable of being produced in this manner. The expanded graphiteparticles that generally are worm-like or vermiform in appearance will,once compressed, maintain the compression set and alignment with theopposed major surfaces of the sheet. Properties of the article may bealtered by coatings and/or the addition of binders or additives prior tothe compression step. See U.S. Pat. No. 3,404,061 to Shane, et al. Thedensity and thickness of the material can be varied by controlling thedegree of compression.

Lower densities are advantageous where surface detail requires embossingor molding, and lower densities aid in achieving good detail. However,higher in-plane strength, thermal conductivity and electricalconductivity are generally favored by more dense sheets. Typically, thedensity of the material will be within the range of from about 0.04g/cm³ to about 1.4 g/cm³.

Graphite material made as described above typically exhibits anappreciable degree of anisotropy due to the alignment of graphiteparticles parallel to the major opposed, parallel surfaces of thematerial, with the degree of anisotropy increasing upon roll pressing toincreased density. In roll-pressed anisotropic material, the thickness,i.e. the direction perpendicular to the opposed, parallel surfacescomprises the “c” direction and the directions ranging along the lengthand width, i.e. along or parallel to the opposed, major surfacescomprises the “a” directions and the thermal, electrical and fluiddiffusion properties of the material are very different, by orders ofmagnitude typically, for the “c” and “a” directions.

DISCLOSURE OF THE INVENTION

It is an object of the invention to provide a resin impregnated graphitearticle suitable for use in electronic thermal management (ETM),supercapacitors or secondary batteries.

It is a further object of this invention to provide graphite structureshaving enhanced in-plane properties.

It is a further object of the invention to provide a machinable graphitestructure having relatively high thermal conductivity in the “a”directions and relatively low conductivity in the “c” direction.

These and other objects are accomplished by the present invention, whichprovides structure comprising resin impregnated graphite articles formedof compressed particles of exfoliated graphite.

BEST MODE FOR CARRYING OUT THE INVENTION

This invention is based upon the finding that when articles of epoxyimpregnated graphite are compressed (such as by calendering) and thencured at elevated temperatures and pressures, the resultant materialexhibits unexpectedly good mechanical and thermal properties and alsopossesses good machinability.

Before describing the manner in which the invention improves currentmaterials, a brief description of graphite and its formation intointegrated articles, which will become the primary substrate for formingthe products of the invention, is in order.

Preparation of Graphite Articles

Graphite is a crystalline form of carbon comprising atoms covalentlybonded in flat layered planes with weaker bonds between the planes. Bytreating particles of graphite, such as natural graphite flake, with anintercalant of, e.g. a solution of sulfuric and nitric acid, the crystalstructure of the graphite reacts to form a compound of graphite and theintercalant. The treated particles of graphite are hereafter referred toas “particles of intercalated graphite.” Upon exposure to hightemperature, the intercalant within the graphite decomposes andvolatilizes, causing the particles of intercalated graphite to expand indimension as much as about 80 or more times its original volume in anaccordion-like fashion in the “c” direction, i.e. in the directionperpendicular to the crystalline planes of the graphite. The exfoliatedgraphite particles are vermiform in appearance, and are thereforecommonly referred to as worms, and are sometimes referred to herein as“particles of expanded graphite.” The worms may be compressed togetherinto articles that, unlike the original graphite flakes, can be formedand cut into various shapes and provided with small transverse openingsby deforming mechanical impact.

Graphite starting materials for the inventive materials include highlygraphitic carbonaceous materials capable of intercalating organic andinorganic acids as well as halogens and then expanding when exposed toheat. These highly graphitic carbonaceous materials most preferably havea degree of graphitization of about 1.0. As used in this disclosure, theterm “degree of graphitization” refers to the value g according to theformula: $g = \frac{3.45 \cdot {d(002)}}{0.095}$where d(002) is the spacing between the graphitic layers of the carbonsin the crystal structure measured in Angstrom units. The spacing dbetween graphite layers is measured by standard X-ray diffractiontechniques. The positions of diffraction peaks corresponding to the(002), (004) and (006) Miller Indices are measured, and standardleast-squares techniques are employed to derive spacing which minimizesthe total error for all of these peaks. Examples of highly graphiticcarbonaceous materials include natural graphites from various sources,as well as other carbonaceous materials such as carbons prepared bychemical vapor deposition and the like. Natural graphite is mostpreferred.

The graphite starting materials for the materials used in the presentinvention may contain non-carbon components so long as the crystalstructure of the starting materials maintains the required degree ofgraphitization and they are capable of exfoliation. Generally, anycarbon-containing material, the crystal structure of which possesses therequired degree of graphitization and which can be exfoliated, issuitable for use with the present invention. Such graphite preferablyhas an ash content of less than twenty weight percent. More preferably,the graphite employed for the present invention will have a purity of atleast about 94%. In the most preferred embodiment, the graphite employedwill have a purity of at least about 98%.

A common method for manufacturing graphite sheets is described by Shaneet al. in U.S. Pat. No. 3,404,061, the disclosure of which isincorporated herein by reference. In one embodiment of the practice ofthe Shane et al. method, natural graphite flakes are intercalated bydispersing the flakes in a solution containing e.g., a mixture of nitricand sulfuric acid, advantageously at a level of about 20 to about 300parts by weight of intercalant solution per 100 parts by weight ofgraphite flakes (pph). The intercalation solution contains oxidizing andother intercalating agents known in the art. Examples include thosecontaining oxidizing agents and oxidizing mixtures, such as solutionscontaining nitric acid, potassium chlorate, chromic acid, potassiumpermanganate, potassium chromate, potassium dichromate, perchloric acid,and the like, or mixtures, such as for example, concentrated nitric acidand chlorate, chromic acid and phosphoric acid, sulfuric acid and nitricacid, or mixtures of a strong organic acid, e.g. trifluoroacetic acid,and a strong oxidizing agent soluble in the organic acid. Alternatively,an electric potential can be used to bring about oxidation of thegraphite. Chemical species that can be introduced into the graphitecrystal using electrolytic oxidation include sulfuric acid as well asother acids.

In a preferred embodiment, the intercalating agent is a solution of amixture of sulfuric acid, or sulfuric acid and phosphoric acid, and anoxidizing agent, i.e. nitric acid, perchloric acid, chromic acid,potassium permanganate, hydrogen peroxide, iodic or periodic acids, orthe like. Although less preferred, the intercalation solution maycontain metal halides such as ferric chloride, and ferric chloride mixedwith sulfuric acid, or a halide, such as bromine as a solution ofbromine and sulfuric acid or bromine in an organic solvent.

The quantity of intercalation solution may range from about 20 to about150 pph and more typically about 50 to about 120 pph. After the flakesare intercalated, any excess solution is drained from the flakes and theflakes are water-washed. Alternatively, the quantity of theintercalation solution may be limited to between about 10 and about 50pph, which permits the washing step to be eliminated as taught anddescribed in U.S. Pat. No. 4,895,713, the disclosure of which is alsoherein incorporated by reference.

The particles of graphite flake treated with intercalation solution canoptionally be contacted, e.g. by blending, with a reducing organic agentselected from alcohols, sugars, aldehydes and esters which are reactivewith the surface film of oxidizing intercalating solution attemperatures in the range of 25° C. and 125° C. Suitable specificorganic agents include hexadecanol, octadecanol, 1-octanol, 2-octanol,decylalcohol, 1, 10 decanediol, decylaldehyde, 1-propanol, 1,3propanediol, ethyleneglycol, polypropylene glycol, dextrose, fructose,lactose, sucrose, potato starch, ethylene glycol monostearate,diethylene glycol dibenzoate, propylene glycol monostearate, glycerolmonostearate, dimethyl oxylate, diethyl oxylate, methyl formate, ethylformate, ascorbic acid and lignin-derived compounds, such as sodiumlignosulfate. The amount of organic reducing agent is suitably fromabout 0.5 to 4% by weight of the particles of graphite flake.

The use of an expansion aid applied prior to, during or immediatelyafter intercalation can also provide improvements. Among theseimprovements can be reduced exfoliation temperature and increasedexpanded volume (also referred to as “worm volume”). An expansion aid inthis context will advantageously be an organic material sufficientlysoluble in the intercalation solution to achieve an improvement inexpansion. More narrowly, organic materials of this type that containcarbon, hydrogen and oxygen, preferably exclusively, may be employed.Carboxylic acids have been found especially effective. A suitablecarboxylic acid useful as the expansion aid can be selected fromaromatic, aliphatic or cycloaliphatic, straight chain or branched chain,saturated and unsaturated monocarboxylic acids, dicarboxylic acids andpolycarboxylic acids which have at least 1 carbon atom, and preferablyup to about 15 carbon atoms, which is soluble in the intercalationsolution in amounts effective to provide a measurable improvement of oneor more aspects of exfoliation. Suitable organic solvents can beemployed to improve solubility of an organic expansion aid in theintercalation solution.

Representative examples of saturated aliphatic carboxylic acids areacids such as those of the formula H(CH₂)_(n)COOH wherein n is a numberof from 0 to about 5, including formic, acetic, propionic, butyric,pentanoic, hexanoic, and the like. In place of the carboxylic acids, theanhydrides or reactive carboxylic acid derivatives such as alkyl esterscan also be employed. Representative of alkyl esters are methyl formateand ethyl formate. Sulfuric acid, nitric acid and other known aqueousintercalants have the ability to decompose formic acid, ultimately towater and carbon dioxide. Because of this, formic acid and othersensitive expansion aids are advantageously contacted with the graphiteflake prior to immersion of the flake in aqueous intercalant.Representative of dicarboxylic acids are aliphatic dicarboxylic acidshaving 2-12 carbon atoms, in particular oxalic acid, fumaric acid,malonic acid, maleic acid, succinic acid, glutaric acid, adipic acid,1,5-pentanedicarboxylic acid, 1,6-hexanedicarboxylic acid,1,10-decanedicarboxylic acid, cyclohexane-1,4-dicarboxylic acid andaromatic dicarboxylic acids such as phthalic acid or terephthalic acid.Representative of alkyl esters are dimethyl oxylate and diethyl oxylate.Representative of cycloaliphatic acids is cyclohexane carboxylic acidand of aromatic carboxylic acids are benzoic acid, naphthoic acid,anthranilic acid, p-aminobenzoic acid, salicylic acid, o-, m- andp-tolyl acids, methoxy and ethoxybenzoic acids, acetoacetamidobenzoicacids and, acetamidobenzoic acids, phenylacetic acid and naphthoicacids. Representative of hydroxy aromatic acids are hydroxybenzoic acid,3-hydroxy-1-naphthoic acid, 3-hydroxy-2-naphthoic acid,4-hydroxy-2-naphthoic acid, 5-hydroxy-1-naphthoic acid,5-hydroxy-2-naphthoic acid, 6-hydroxy-2-naphthoic acid and7-hydroxy-2-naphthoic acid. Prominent among the polycarboxylic acids iscitric acid.

The intercalation solution will be aqueous and will preferably containan amount of expansion aid of from about 1 to 10%, the amount beingeffective to enhance exfoliation. In the embodiment wherein theexpansion aid is contacted with the graphite flake prior to or afterimmersing in the aqueous intercalation solution, the expansion aid canbe admixed with the graphite by suitable means, such as a V-blender,typically in an amount of from about 0.2% to about 10% by weight of thegraphite flake.

After intercalating the graphite flake, and following the blending ofthe intercalant coated intercalated graphite flake with the organicreducing agent, the blend is exposed to temperatures in the range of 25°to 125° C. to promote reaction of the reducing agent and intercalantcoating. The heating period is up to about 2 hours, with shorter heatingperiods, e.g., at least about 10 minutes, for higher temperatures in theabove-noted range. Times of one-half hour or less, e.g., on the order of10 to 25 minutes, can be employed at the higher temperatures.

The above described methods for intercalating and exfoliating graphiteflake may beneficially be augmented by a pretreatment of the graphiteflake at graphitization temperatures, i.e. temperatures in the range ofabout 3000° C. and above and by the inclusion in the intercalant of alubricious additive.

The pretreatment, or annealing, of the graphite flake results insignificantly increased expansion (i.e., increase in expansion volume ofup to 300% or greater) when the flake is subsequently subjected tointercalation and exfoliation. Indeed, desirably, the increase inexpansion is at least about 50%, as compared to similar processingwithout the annealing step. The temperatures employed for the annealingstep should not be significantly below 3000° C., because temperatureseven 100° C. lower result in substantially reduced expansion.

The annealing of the present invention is performed for a period of timesufficient to result in a flake having an enhanced degree of expansionupon intercalation and subsequent exfoliation. Typically the timerequired will be 1 hour or more, preferably 1 to 3 hours and will mostadvantageously proceed in an inert environment. For maximum beneficialresults, the annealed graphite flake will also be subjected to otherprocesses known in the art to enhance the degree expansion—namelyintercalation in the presence of an organic reducing agent, anintercalation aid such as an organic acid, and a surfactant washfollowing intercalation. Moreover, for maximum beneficial results, theintercalation step may be repeated.

The annealing step of the instant invention may be performed in aninduction furnace or other such apparatus as is known and appreciated inthe art of graphitization; for the temperatures here employed, which arein the range of 3000° C., are at the high end of the range encounteredin graphitization processes.

Because it has been observed that the worms produced using graphitesubjected to pre-intercalation annealing can sometimes “clump” together,which can negatively impact area weight uniformity, an additive thatassists in the formation of “free flowing” worms is highly desirable.The addition of a lubricious additive to the intercalation solutionfacilitates the more uniform distribution of the worms across the bed ofa compression apparatus (such as the bed of a calender stationconventionally used for compressing, or “calendering,” graphite wormsinto an integrated graphite article). The resulting article thereforehas higher area weight uniformity and greater tensile strength. Thelubricious additive is preferably a long chain hydrocarbon, morepreferably a hydrocarbon having at least about 10 carbons. Other organiccompounds having long chain hydrocarbon groups, even if other functionalgroups are present, can also be employed.

More preferably, the lubricious additive is an oil, with a mineral oilbeing most preferred, especially considering the fact that mineral oilsare less prone to rancidity and odors, which can be an importantconsideration for long term storage. It will be noted that certain ofthe expansion aids detailed above also meet the definition of alubricious additive. When these materials are used as the expansion aid,it may not be necessary to include a separate lubricious additive in theintercalant.

The lubricious additive is present in the intercalant in an amount of atleast about 1.4 pph, more preferably at least about 1.8 pph. Althoughthe upper limit of the inclusion of lubricous additive is not ascritical as the lower limit, there does not appear to be any significantadditional advantage to including the lubricious additive at a level ofgreater than about 4 pph.

The thus treated particles of graphite are sometimes referred to as“particles of intercalated graphite.” Upon exposure to high temperature,e.g. temperatures of at least about 160° C. and especially about 700° C.to 1200° C. and higher, the particles of intercalated graphite expand asmuch as about 80 to 1000 or more times their original volume in anaccordion-like fashion in the c-direction, i.e. in the directionperpendicular to the crystalline planes of the constituent graphiteparticles. The expanded, i.e. exfoliated, graphite particles arevermiform in appearance, and are therefore commonly referred to asworms. The worms may be compressed together into articles that, unlikethe original graphite flakes, can be formed and cut into various shapesand provided with small transverse openings by deforming mechanicalimpact as hereinafter described.

The graphite materials prepared as described are coherent, with goodhandling strength, and are suitably compressed, e.g. by molding orroll-pressing, to a thickness of about 0.075 mm to 30 mm and a typicaldensity of about 0.1 to 1.5 grams per cubic centimeter (g/cc). Fromabout 1.5-30% by weight of ceramic additives can be blended with theintercalated graphite flakes as described in U.S. Pat. No. 5,902,762(which is incorporated herein by reference) to provide enhanced resinimpregnation in the final graphite product. The additives includeceramic fiber particles having a length of about 0.15 to 1.5millimeters. The width of the particles is suitably from about 0.04 to0.004 mm. The ceramic fiber particles are non-reactive and non-adheringto graphite and are stable at temperatures up to about 1100° C.,preferably about 1400° C. or higher. Suitable ceramic fiber particlesare formed of macerated quartz glass fibers, carbon and graphite fibers,zirconia, boron nitride, silicon carbide and magnesia fibers, naturallyoccurring mineral fibers such as calcium metasilicate fibers, calciumaluminum silicate fibers, aluminum oxide fibers and the like.

As noted above, the graphite materials are also treated with resin andthe absorbed resin, after curing, enhances the moisture resistance andhandling strength, i.e. stiffness, of the material as well as “fixing”the morphology of the sheet. The amount of resin within the epoxyimpregnated graphite articles should be an amount sufficient to ensurethat the final cured structure is dense and cohesive, yet theanisotropic thermal conductivity associated with a densified graphitestructure is preserved or improved. Suitable resin content is preferablyat least about 3% by weight, more preferably about 5 to 35% by weight,and suitably up to about 60% by weight. Resins found especially usefulin the practice of the present invention include acrylic-, epoxy- andphenolic-based resin systems, fluoro-based polymers, or mixturesthereof. Suitable epoxy resin systems include those based on diglycidylether of bisphenol A (DGEBA) and other multifunctional resin systems;phenolic resins that can be employed include resole and novolacphenolics. Optionally, the flexible graphite may be impregnated withfibers and/or salts in addition to the resin or in place of the resin.Additionally, reactive or non-reactive additives may be employed withthe resin system to modify properties (such as tack, material flow,hydrophobicity, etc.).

In a typical resin impregnation step, the flexible graphite material ispassed through a vessel and impregnated with the resin system from, e.g.spray nozzles, the resin system advantageously being “pulled through themat” by means of a vacuum chamber. Typically, but not necessarily, theresin system is solvated to facilitate application into the flexiblegraphite. The resin is thereafter preferably dried, reducing the tack ofthe resin and the resin-impregnated article.

Alternatively, the flexible graphite of the present invention mayutilize particles of reground flexible graphite materials rather thanfreshly expanded worms. The reground materials may be newly formedmaterial, recycled material, scrap material, or any other suitablesource.

Also the processes of the present invention may use a blend of virginmaterials and recycled materials.

The source material for recycled materials may be articles or trimmedportions of articles that have been compression molded as describedabove, or sheets that have been compressed with, for example,pre-calendering rolls, but have not yet been impregnated with resin.Furthermore, the source material may be impregnated with resin, but notyet cured, or impregnated with resin and cured. The source material mayalso be recycled flexible graphite fuel cell components such as flowfield plates or electrodes. Each of the various sources of graphite maybe used as is or blended with natural graphite flakes.

Once the source material of flexible graphite is available, it can thenbe comminuted by known processes or devices, such as a jet mill, airmill, blender, etc. to produce particles. Preferably, a majority of theparticles have a diameter such that they will pass through 20 U.S. mesh;more preferably a major portion (greater than about 20%, most preferablygreater than about 50%) will not pass through 80 U.S. mesh. Mostpreferably the particles have a particle size of no greater than about20 mesh. It may be desirable to cool the flexible graphite when it isresin-impregnated as it is being comminuted to avoid heat damage to theresin system during the comminution process.

The size of the comminuted particles may be chosen so as to balancemachinability and formability of the graphite article with the thermalcharacteristics desired. Thus, smaller particles will result in agraphite article which is easier to machine and/or form, whereas largerparticles will result in a graphite article having higher anisotropy,and, therefore, greater in-plane electrical and thermal conductivity.

Once the source material is comminuted (if the source material has beenresin impregnated, then preferably the resin is removed from theparticles), it is then re-expanded. The re-expansion may occur by usingthe intercalation and exfoliation process described above and thosedescribed in U.S. Pat. No. 3,404,061 to Shane et al. and U.S. Pat. No.4,895,713 to Greinke et al.

Typically, after intercalation the particles are exfoliated by heatingthe intercalated particles in a furnace. During this exfoliation step,intercalated natural graphite flakes may be added to the recycledintercalated particles. Preferably, during the re-expansion step theparticles are expanded to have a specific volume in the range of atleast about 100 cc/g and up to about 350 cc/g or greater. Finally, afterthe re-expansion step, the re-expanded particles may be compressed intocoherent materials and impregnated with resin, as described.

Graphite materials prepared according to the foregoing description canalso be generally referred to as compressed particles of exfoliatedgraphite. Since the materials are resin-impregnated, the resin in thesheets needs to be cured before the sheets are used in their intendedapplications, such as for electronic thermal management.

According to the invention, resin-impregnated graphite materialsprepared as described above are compressed to the desired thickness andshape, commonly a thickness of about 0.35 mm to 0.5 mm, at which timethe impregnated mats have a density of about 1.4 g/cm³ to about 1.9g/cm³.

One type of apparatus for continuously forming resin-impregnated andcompressed flexible graphite materials is shown in InternationalPublication No. WO 00/64808, the disclosure of which is incorporatedherein by reference.

Following the compression step (such as by calendering), the impregnatedmaterials are cut to suitable-sized pieces and placed in a press, wherethe resin is cured at an elevated temperature. The temperature should besufficient to ensure that the lamellar structure is densified at thecuring pressure, while the thermal properties of the structure are notadversely impacted. Generally, this will require a temperature of atleast about 90° C., and generally up to about 200° C. Most preferably,cure is at a temperature of from about 150° C. to 200° C. The pressureemployed for curing will be somewhat a function of the temperatureutilized, but will be sufficient to ensure that the lamellar structureis densified without adversely impacting the thermal properties of thestructure. Generally, for convenience of manufacture, the minimumrequired pressure to densify the structure to the required degree willbe utilized. Such a pressure will generally be at least about 7megapascals (Mpa, equivalent to about 1000 pounds per square inch), andneed not be more than about 35 Mpa (equivalent to about 5000 psi), andmore commonly from about 7 to about 21 Mpa (1000 to 3000 psi). Thecuring time may vary depending on the resin system and the temperatureand pressure employed, but generally will range from about 0.5 hours to2 hours. After curing is complete, the composites are seen to have adensity of at least about 1.8 g/cm³ and commonly from about 1.8 g/cm³ to2.0 g/cm³.

Although the formation of sheets through calendering or molding is themost common method of formation of the graphite materials useful in thepractice of the present invention, other forming methods can also beemployed. For instance, the exfoliated graphite particles can becompression molded into a net shape or near net shape. Thus, if the endapplication requires an article, such as a heat sink or heat spreader,assuming a certain shape or profile, that shape or profile can be moldedinto the graphite article, before or after resin impregnation. Curewould then take place in a mold assuming the same shape; indeed, in thepreferred embodiment, compression and curing will take place in the samemold. Machining to the final shape can then be effected.

The temperature- and pressure-cured graphite/resin composites of thepresent invention provide for the first time a graphite-based compositematerial having in-plane thermal conductivity rivaling or exceeding thatof copper, at a fraction of the weight of copper. More specifically, theinventive composites exhibit in-plane thermal conductivities of at leastabout 300 W/mK, with through-plane thermal conductivities of less thanabout 15 W/mK, more preferably less than about 10 W/mK. Such materialswill be extremely useful in heat dissipation applications, such as inheat sinks, heat spreaders, heat pipes and the like, especially wherethe weight of copper would be disadvantageous.

The above description is intended to enable the person skilled in theart to practice the invention. It is not intended to detail all of thepossible variations and modifications that will become apparent to theskilled worker upon reading the description. It is intended, however,that all such modifications and variations be included within the scopeof the invention that is defined by the following claims. The claims areintended to cover the indicated elements and steps in any arrangement orsequence that is effective to meet the objectives intended for theinvention, unless the context specifically indicates the contrary.

1-15. (canceled)
 16. A method of forming a resin/graphite compositecomprising impregnated a graphite article with resin and curing theresin under pressure and at an elevated temperature.
 17. The method ofclaim 16 wherein the resin is an epoxy.
 18. The method of claim 16wherein the graphite article is pressure cured at a temperature of atleast about 90° C. and at a pressure of at least about 7 Mpa.
 19. Themethod of claim 16 wherein the density of the cured composite is greaterthan about 1.8 g/cm³.
 20. The method of claim 16 wherein the graphitearticle is pressure cured at a temperature of below about 200° C. and ata pressure of below about 35 Mpa.